This invention relates to a memory apparatus using insulated gate field effect transistors which constitute nonvolatile memory cell elements.
A nonvolatile memory cell constituted of variable threshold insulated gate field effect transistors (hereinafter referred to merely as MISFET's) having a specific gate structure retains its memory content even when a power supply is interrupted. A memory apparatus is well known in which such a nonvolatile memory cell constituted of MISFET's is used for each volatile memory flip-flop of an ordinary memory and when a power supply is interrupted a memory content on a volatile memory flip-flop is transferred to the nonvolatile memory cell. Such a conventional memory apparatus is generally designed to store one-bit information per memory cell and it is therefore impossible to independently store a plurality of information in one memory cell.